When mapreducing, a strange error appear sometimes. Here is the stacktrace :
System.InvalidOperationException: Duplicate element name 'cause'. at MongoDB.Bson.BsonDocument.Add(BsonElement element) in C:\work\10gen\mongodb\mongo-csharp-driver\Bson\ObjectModel\BsonDocument.cs:line 453
at MongoDB.Bson.BsonDocument.Deserialize(BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options) in C:\work\10gen\mongodb\mongo-csharp-driver\Bson\ObjectModel\BsonDocument.cs:line 720
at MongoDB.Bson.BsonDocument.ReadFrom(BsonReader bsonReader) in C:\work\10gen\mongodb\mongo-csharp-driver\Bson\ObjectModel\BsonDocument.cs:line 393
at MongoDB.Bson.Serialization.BsonSerializer.Deserialize(BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options) in C:\work\10gen\mongodb\mongo-csharp-driver\Bson\Serialization\BsonSerializer.cs:line 214
at MongoDB.Driver.Internal.MongoReplyMessage`1.ReadFrom(BsonBuffer buffer, IBsonSerializationOptions serializationOptions) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Internal\MongoReplyMessage.cs:line 94
at MongoDB.Driver.Internal.MongoConnection.ReceiveMessage[TDocument](BsonBinaryReaderSettings readerSettings, IBsonSerializationOptions serializationOptions) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Internal\MongoConnection.cs:line 386
at MongoDB.Driver.MongoCursorEnumerator`1.GetReply(MongoConnection connection, MongoRequestMessage message) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCursorEnumerator.cs:line 263
at MongoDB.Driver.MongoCursorEnumerator`1.GetFirst() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCursorEnumerator.cs:line 223
at MongoDB.Driver.MongoCursorEnumerator`1.MoveNext() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCursorEnumerator.cs:line 126
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source) at MongoDB.Driver.MongoCollection.FindOneAs[TDocument](IMongoQuery query) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCollection.cs:line 498 at MongoDB.Driver.MongoCollection`1.FindOne(IMongoQuery query) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCollection.cs:line 1639 at MongoDB.Driver.MongoDatabase.RunCommandAs(Type commandResultType, IMongoCommand command) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoDatabase.cs:line 891
at MongoDB.Driver.MongoDatabase.RunCommandAs[TCommandResult](IMongoCommand command) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoDatabase.cs:line 866
at MongoDB.Driver.MongoCollection.MapReduce(BsonJavaScript map, BsonJavaScript reduce, IMongoMapReduceOptions options) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCollection.cs:line 1106 at MongoDB.Driver.MongoCollection.MapReduce(IMongoQuery query, BsonJavaScript map, BsonJavaScript reduce, IMongoMapReduceOptions options) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCollection.cs:line 1127

I think this due to the driver incapicity to add several times the same field to a bsondocument.